Intergranular stress corrosion

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Proceedings 1987 EPRI Workshop on Mechanisms of Primary Water Intergranular Stress Corrosion Crackiag," Report NP-5987M, Electric Power Research Institute, Palo Alto, Calif., Sept. 1988. 

Fig. 2-17 Relation between the time to failure by intergranular stress corrosion cracking and potential for tensile specimens of soft iron (a) boiling 55% Ca(N03)2 solution, 5 = 0.65 R a = 0.90 R (b) 33% NaOH, a = 300 N mm, at various temperatures.

Fig. 2-18 J U) curves and critical potential range for intergranular stress corrosion (hatched) for a hardened 10 CrMo 9 10 steel (ASTM P21) in boiling 35% NaOH — potentio-dynamically measured with +0.6 V h - - potential change after every 0.5 h At/ = +0.1 V x-x-x potential change after every 0.5 hAf/ = -0.1 V.

With materials with high resistance to intergranular stress corrosion, f/ can be more positive in group I and more negative in group IV. Corrosion susceptibility increases with rising temperature (see Chapter 21). 

Intergranular stress-corrosion cracking (IG-SCC) can occur in some sensitized materials when placed under tensile stress. Thus DL-EPR has been used to study the effects of aging time on the susceptibility of Alloy 600 to IGSCC, as shown in Fig. 41 (39). This work also shows the need to modify the experimental parameters of the test to achieve optimal correlation for alloys other than Type 304SS, in this case lowering the KCNS concentration and the temperature while raising the peak potential and the scan rate. 

Reactors are normally made of low-alloy steel (selected per API Publication 941) and clad or weld overlaid with type 347 (UNS S34700) SS. The stabilized grades (i.e., type 947 or 321 [UNS S32100] SS) are normally used for all stainless equipment in these units to avoid intergranular stress corrosion cracking (SCC) during downtime. Downtime corrosion is discussed in the next section of this chapter, Feed-Effluent Exchangers."... 

Type 304 is listed because it Is the least costly of the acceptable materials. Other 300 series SS s may be needed for considerations other than low temperature. For example, low-carbon grades are desirable for seacoast environments to avoid intergranular stress corrosion cracking during the periods when the material is not at cryogenic temperature,... 

As a second example, we show in Fig. 27 the nucleation of intergranular stress corrosion cracks in sensitized Type 304SS as observed by Diercks and Dragel at the Argonne National Laboratory (see [56]). The cracks were found to nucleate from pits or at intergranular penetrations (e.g., intergranular attack, IGA). In other cases, cracks are found to nucleate from MnS inclusion but, in this case too, the primary event is probably the formation of a pit which acts as a stress riser. Examination of many micrographs indicates that the critical nucleus is of the order of 30-1 (X) pm deep. 

M. E. Indig, B. M. Gordon, R. B. Davis, J. E. Weber, Evaluation of In-Reactor Intergranular Stress Corrosion Cracking via Electrochemical Measurements, in Proc. 2nd Int. Symp. Environ. Degr. Mater. Nucl. Power Systs. - Water reactors, Monterey, CA, Sept. 9-12, 1985, American Nuclear Society, LaGrange Park, IL (1986). 

Since 1973 [3] and 1975 in Dresden 2 [4], on line measurements of redox potential have been performed in the BWRs (Boiling Water Reactors) primary coolant in order to verify the IGSCC (Intergranular Stress Corrosion Craking) susceptibility of austenitic stainless steel components. Such measurements turned out to be fundamental for estimating the effect of hydrogen addition as a remedy for IGSCC [5-6]. 

T.M. Devine and B.J. Drummond, Use of Accelerated Intergranular Corrosion Tests and Pitting Corrosion Tests to Detect Sensitization and Susceptibility to Intergranular Stress Corrosion Cracking in High Temperature Water of Duplex 308 Stainless Steel, Corrosion, Vol 37, 1981, p 104-115... 

As with all standardized tests (e g., the ASTM A 262 procedures previously discussed), correlations must be established between the EPR Pa values and service performance. For example, a criterion of Pa < 2 C/cm2 has been proposed for adequate resistance to intergranular corrosion leading to intergranular stress-corrosion cracking (IGSCC) of type 304 and 304L pipe and welds. Other limits would be set depending on the material, application, and environment (Ref 105, 106). 

Fig. 7.90 Effect of stressing direction on the intergranular stress-corrosion crack path in susceptible high-strength aluminum alloy. Dark boundaries are representative of ones favored for cracking for indicated direction of applied stress. Source Ref 97...

M. Hishida and H. Nakada, Critical Cooling Rate of 18Cr-8Ni Stainless Steel for Sensitization and Subsequent Intergranular Stress Corrosion Cracking, Corrosion, Vol 34, 1978, p 338-344... 

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